Light emitting diodes (LEDs) are replacing conventional light sources such as fluorescent and incandescent light bulbs in many applications. LEDs have similar electrical efficiency and longer lifetimes than fluorescent light sources. In addition, the driving voltages needed are compatible with the battery power available on many portable devices.
An LED light source typically includes one or more semiconductor dies that are mounted on a substrate that provides the power to the LED and a structure that can be utilized to mount the light source on the appropriate surface of the final product in which the light source is to function. For a number of reasons, the LED is typically covered with an encapsulating layer that has a spherical outer surface. First, the die must be protected from the environment.
Second, some light sources utilize a layer of phosphor that converts all or a portion of the light from the LED to light having a different spectrum such that the resultant light is perceived to be of the desired color. So-called “white” LEDs are constructed in this manner. The phosphor is typically provided as a powder that is suspended in the encapsulating layer.
Third, the efficiency of conversion of electrical power to light that is delivered by the LED is significantly improved if the LED is encapsulated in a layer of material that has an index of refraction that is intermediate between that of the material from which the LED layers are constructed and that of the surrounding air. Ideally, this layer has a curved outer surface with a radius of curvature that is sufficient to assure that the light leaving the top surface of the LED strikes the surface at an angle that is less than the critical angle.
For lighting applications, cost is also of primary concern. One method for providing the encapsulating layer that has the potential for providing a low cost utilizes a droplet of silicone that is deposited over the die after the die has been connected to an underlying substrate. The viscosity of the silicone is sufficient to assure that the droplet will solidify to the desired shape before the material spreads out excessively on the surface of the substrate in question. In addition, silicone has a number of properties that make it a good carrier for phosphors in phosphor converted light sources. In particular, silicone is resistant to damage by short wavelength light. Many phosphor-converted light sources utilize blue or UV light to excite the phosphors.
However, providing a reproducible encapsulation structure that does not entail utilizing some form of separate containment dam to prevent the silicone from spreading before it sets remains a challenge in light sources that do not utilize a reflector cup around the LED. If the light source includes a reflector cup that collects the light leaving the sides of the LED and redirects that light into the forward direction, the cup can act as a mold for the silicone encapsulation layer. However, in many applications, the silicone encapsulant is applied before the reflector is bonded to the light source. In addition, light sources that lack the reflectors, either because of cost or a need to maintain a low profile, must utilize some other means to provide reproducible encapsulation domes.